Apparatus for testing a nitrogen-gas-stabilized cement

ABSTRACT

A testing apparatus is provided for measuring changes in the bulk volume and gas permeability of a cement slurry, such as one formed by dissolving in the slurry mix water a correlated amount of gas-forming reactants having a delayed rate of reaction such that the gas production occurs mainly at the time the volume of the slurry mix water is being reduced to a low value during the setting and hardening of the cement.

RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.227,027 filed Jan. 21, 1981 now U.S. Pat. No. 4,333,764.

BACKGROUND OF THE INVENTION

This invention relates to making and using an aqueous slurry ofhydraulic cement. More particularly, it relates to providing such aslurry which contains nitrogen gas-forming reactants havingconcentrations and reaction rates which are correlated with thetemperature and pressure in the zone to be cemented to provide avolume-stabilizing amount of gas production during substantially all ofthe setting and hardening of the cement.

Numerous patents and publications have appeared regarding foamed cementsand their preparation and use. Such cements have been said to be usefulas lightweight cements, construction materials, grouting materials,thermal insulating materials, non-shrinking cements, gas-imperviouscements, fluid-permeable cements, and the like. The procedures formaking and using such foamed or non-shrinking cements have includedmodifying conventional cement slurries by adding various hydrogengenerating reactants, nitrogen generating reactants of numerous kinds,oxygen-generating reactants, water-swellable materials, and the like.

U.S. Pat. No. 2,163,207 relates to making cellular building materials bycombining lime base binders with aqueous ammonia and hypochlorites (andoptionally, peroxides) to produce gaseous nitrogen.

U.S. Pat. No. 2,191,555 relates to making porous cementous buildingmaterial by adding to non-alkaline formulations (such as calcined gypsumplasters) an amide (such as urea) and a material which forms nitrousacid (such as a nitrite salt).

U.S. Pat. No. 2,288,556 relates to making a permeable cement by addingto a hydraulic cement slurry enough gas-generating material (such aspowdered aluminum, calcium carbide, ammonium nitrite, or the like) toform a network of interconnected gas bubbles.

U.S. Pat. No. 2,371,928 relates to making porous materials fromslurries, of cement gypsum or the like, by first mixing the cement withwater, hypochlorite and filling material, then adding a pore-stabilizingsubstance (such as a soap or glue) along with hydrogen peroxide togenerate gaseous oxygen.

U.S. Pat. No. 3,420,299 relates to preventing shrinkage during thesetting of a cement slurry by adding pellets of a water-swellingmaterial (such as bentonite) encapsulated in a material (such as agel-like mud) to be ruptured when the cement slurry begins to harden.

U.S. Pat. No. 3,591,394 relates to making porous cement by adding to thecement slurry a "nitrogen delivering" compound (such as a diazoniumcompound) and an activator (such as sodium aluminate, potassium sulphatelead peroxide, sodium perbolate or the like) and mentions thedesirability of delaying the onset of gas production until the slurryhas been pumped into place.

U.S. Pat. No. 3,926,257 relates to making a foamed cement which isimpermeable to gas by adding a foaming surfactant to the cement slurryso that any inflowing gas will form an immobile foam within the settingcement.

U.S. Pat. No. 4,142,909 relates to making a non-shrinking cement byadding to a hydraulic cement slurry a gas-generating compound which iscapable of a controlled reaction without activators (such asazodicarbonamide, sodium azodicarboxylate, an organic peroxide, orsodium borohydride).

Numerous technical journal publications relate to the problems caused bygas leaking through a cemented annulus of a well borehole.

An article G. Carter and K. Slagle (of Halliburton) "A Study ofCompletion Practices to Minimize Gas Communication", September, 1972,JPT, page 1170, indicated that the problem was first recognized as beingsignificant in the middle 1960s and two important factors for preventingthe leakage are the maintenance of both a hydrostatic head greater thanthe gas pressure and a low fluid loss within the cement slurry.

An article by W. W. Christian, J. Chatterji and G. W. Ostroot ofHalliburton, "Gas Leakage in Primary Cementing--A Field Study andLaboratory Investigation", November 1976, JPT, page 1361, mentioned thatwith deeper well completions across gas-producing horizons the problemof gas leakage was then a major concern and concluded that the lack offluid loss control may be a primary cause of such leakage.

SPE Paper 8255, September 1979, "Annular Gas Flow After Cementing: ALook at Practical Solutions" (by representatives of Exxon and Texas A&M)concluded that a hydraulic pressure loss during the hydration reactionwithin the cement slurry is a primary cause of such leakage and themeans for preventing it include cementing in short stages with slurrieshaving different curing rates or using a foamed slurry containing gasbubbles that act as compressible pressure compensators during thehydration reaction.

SPE Paper 8257, September 1979, "Study of Factors Causing Annular GasFlow Following Primary Cementing" (by representatives of Halliburton)indicated that the problems due to the pressure loss during the cementhydration can be removed by "an entrainment, addition, or in situgeneration of a gas".

SPE Paper 8259, September 1979, "Flow After Cementing-A Field andLaboratory Study" (by representatives of Mobil and B-J Hughes) indicatedthat a cement slurry free water content of any significant magnitude isa significant factor (particularly in inclined boreholdes) capable ofcausing a gas flow after cementing.

Thus, the relatively recent journal publications indicate thedesirability of having gas bubbles distributed uniformly within a cementslurry at the time the slurry is losing its ability to provide ahydrostatic pressure exceeding the pressure in an adjoininggas-containing reservoir. And, the issued patents show that it has longbeen known that numerous reactive compounds and procedures are availablefor causing numerous kinds of gas bubbles to be present within a cementslurry.

However, it is also known that (1) too little gas will provideinsufficient compensation for the pressure loss during the hydration ofthe cement, (2) too much gas will cause the cement to have too muchpermeability and/or too little strength, (3) too early an inclusion ofgas within the cement slurry will allow the gas to become non-uniformlydistributed with respect to the other components of the slurry (whichare much heavier and/or much more viscous than the gas), (4) too late aninclusion of the gas will fail to keep the reservoir gas from enteringand channeling through the hydrating slurry before the cement has setand (5) the inclusion of a highly explosive gas such as hydrogen or acombustion-inducing gas such as oxygen will create a chemical situationwhich is difficult or dangerous to handle. And thus, the teachings ofprior patents and publications are clear that a method for causing somesort of gas formation is desirable--but also show that no such methodwhich is free of disadvantages that may be difficult or impossible toovercome has yet been disclosed.

SUMMARY OF THE INVENTION

The present invention relates to a pumpable aqueous slurry of hydrauliccement which contains, within its aqueous liquid phase,nitrogen-gas-generating reactants which (a) consist of compounds thatreact while dissolved in that aqueous liquid (b) have a rate of reactionand a concentration within the aqueous liquid such that the rate atwhich they generate gas is relatively insignificant until theirconcentration within the aqueous liquid is increased by the loss ofaqueous liquid which accompanies the setting of the cement and (c) arepresent within said aqueous slurry of cement in a proportion such thatthe volume of the gas generated by the reactants at least substantiallyequals the volume lost by the slurry during the setting of the cement.

In one preferred embodiment, the present invention relates to a processfor compounding and emplacing a hydraulic cement within a selected zoneto be cemented. A pumpable aqueous slurry of hydraulic cement componentsis compounded to form a slurry capable of being flowed into and curedwithin the zone to be cemented. Water-soluble reactant compounds, whichare capable of reacting, while dissolved in an aqueous liquid, toproduce nitrogen gas and by-products which are substantially inert tothe components of the cement slurry and the cement it forms, aredissolved within the aqueous liquid phase of the slurry. The totalconcentration of the reactants within the cement slurry is correlatedwith the pressure and temperature within the zone to be cemented sothat, at those conditions, the volume of the gas produced by thereactants substantially equals the volume lost by the slurry during thesetting (or curing) of the cement. The rate at which the reactants reactwithin said aqueous liquid is correlated with the temperature in thezone to be cemented so that, at that temperature and at the initialconcentration of the reactants in the aqueous liquid, the amount of gaswhich they can produce between the times of the compounding of theslurry and the initial setting of the cement is significantly less thanthe total amount they are capable of producing. The so-compounded slurryis then promptly flowed into the zone to be cemented and maintainedsubstantially quiescent. The volume of the aqueous liquid phase of theslurry is reduced during the setting of the cement. This concentratesthe reactants within the remaining aqueous liquid and increases the rateat which the reactants generate gas. As a result of the gas generation,the solidifying cement becomes both impermeable and permeated withnitrogen gasfilled microsized clefts or creases and the bulk volume ofthe setting and set cement remains substantially equal to that of theslurry.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an apparatus for testing thebehavior of a column of cement slurry relative to an inflow of gas andthe transmission of pressure through the slurry.

FIG. 2 shows variations with time in the flow of an externallypressurized gas into and out of a column of cement slurry while thecement is setting.

FIG. 3 shows the percent decrease with time in the volume of a cementslurry during the setting and hardening of the cement.

FIGS. 4 and 5 show variations with time in the flow of externallypressurized gas into and out of a column of cement slurry while thecement is setting and hardening.

FIG. 6 shows percent decrease with time in the volume of a cement slurryduring the setting and hardening of the cement.

FIG. 7 shows the volume changes with time of various cement slurrieswhile the cements are setting and hardening at various temperatures.

DESCRIPTION OF THE INVENTION

It has now been discovered that certain types of nitrogen generatingreactive compounds can be used in a novel way in order to form anaqueous slurry of hydraulic cement that will solidify without shrinking.Such a slurry can be formed by using reactants which react while theyare dissolved within the aqueous liquid phase of a cement slurry. Theconcentration of those reactants within that aqueous liquid and withinthe cement slurry as a whole should be correlated with the pressure andtemperature of the zone in which the cement is to be solidified. Thetotal concentration of those reactants should be such that the volume ofthe gas they generate is capable of substantially matching the volumelost by the slurry during the setting of the cement. In addition, therate at which those reactants react to form the gas should be kept slowuntil the beginning of the initial setting of the cement. At that time,the rate of the gas-production should be increased so that the volume ofthe gas is kept substantially equal to the volume lost by the slurry.Where such reactants are aqueous liquid-phase solutes, theirconcentration is increased by the loss of the solvent liquid. And, ithas now been discovered, the rate of that increase in concentration iscapable of increasing the rate of gas production to the rate needed forstabilizing the volume of the slurry and the cement it forms.

The present process causes a pumpable liquid slurry (which is initiallycapable of transmitting the full hydrostatic pressure of a column ofliquid) to be converted to an initially and finally set cement having agas-stabilized bulk volume substantially equalling that of the slurry.This stabilization is due to the forming of nitrogen-gas-filledmicrosized clefts or creases within the body of the solidified cement.Since the amount of gas which is so-contained is very small, such agas-stabilization is not likely to cause any significant loss ofcompressive strength or density nor any significant increase inpermeability. No such losses or permeability increases have beenindicated by the testing of the invention.

Testing System

FIG. 1 shows a cement slurry testing apparatus. The apparatus includes apressure-resistant U-tube 1. The upper portions of the U-tube legs areconnected to pressure gauges 2 and 3. The left leg is connected to avalved conduit 4 leading to a gas source (not shown). A branch 5 of theconduit 4 extends into a standpipe 6 having a lower end connected,through a valved line 7, to a water pump 8. The water pump intake isconnected to a water reservoir (not shown). The lower end of thegas/water standpipe 6 is also connected to a valved line 9 leading to awater discharge.

In measuring the properties of a cement slurry, 10, a sample of theslurry is placed in the U-tube. In the right hand leg, a body of oil 11is placed between the top of the cement slurry and the pressure gauge 2.An inert gas is injected to form a gas column 12(a) in the upper portionof the standpipe 6 and a gas column 12(b) in the upper portion of theleft leg of the U-tube. The gas is pressurized to the pressure selectedfor the testing of the cement slurry.

The cement slurry is kept static within the U-tube, at the selectedtemperature and in contact with the externally pressurized gas(preferably nitrogen) at the selected pressure, throughout at least theinitial setting or curing of the cement. The water pump 8 is operated tomaintain the selected pressure, for example, by repetitively injectingor allowing a backflowing of enough water to maintain the selected gaspressure. For example, the pump may inject water when the pressure fallsto 310 and stop when it rises to 340 psig.

In such a procedure, with the pressure being kept substantiallyconstant, the volume of water which is injected corresponds to thevolume of gas which has flowed through the top of the gas-containing legof the U-tube and into the column of cement slurry. The volume of thisgas can readily be measured by measuring the amount of water withdrawnfrom the water reservoir, and/or by measuring the amount of waterinjected into the standpipe 6. The volume of any gas which is generatedwithin the cement slurry at a rate sufficient to increase the gaspressure can similarly be measured by measuring the amount of water thatis displaced back through the pump. The pressure gauge 2 monitors anyvariation in the pressure in the oil-containing leg of the U-tube, whilethe gauge 3 similarly monitors the pressure in the gas-containing leg.

Substantially any alternative testing chamber can be used in place ofthe U-tube 1 and associated equipment shown in FIG. 1 as long as thetest chamber is arranged so that a sample of cement slurry can bepositioned within the chamber so that it fills a rigid-walled tubularportion while a body of gas is maintained within the chamber so that itis in contact with and confined by the slurry and means are provided formaintaining the gas and slurry at a selected temperature and pressureand measuring the amount of gas which flows into the space initiallyoccupied by the cement slurry. For use in measurements at relativelyhigh pressures, it may be desirable to avoid the need for a verythick-walled chamber by surrounding the whole of the testing chamberwith a liquid-filled vessel which is capable of safely confining the gaspressure to be used. It is convenient to dispose a body of mobile liquidsuch as the oil column 11 of FIG. 1 on the opposite side of the portionof cement slurry which is in contact with a body of pressurized gas inorder to facilitate the bleeding down of the fluid pressure within thetest chamber on the side of the cement slurry opposite the sidecontacted by the gas so that a gas pressure gradient is formed acrossthe body of the set cement, in order to determine the rate ofgas-leakage. In general, such a mobile fluid can be substantially anywhich has a relatively low viscosity and does not affect curingproperties of the cement. Similarly, the pressurized gas can besubstantially any which is inert to the curing of the cement and has PVTproperties making it feasible to determine the volume changes within thecement slurry which correspond to a gas flow into or out of the spaceinitially occupied by the cement slurry.

Gas Flow Control at 72° F. and 340 psig

FIG. 2 illustrates a plot of the amount of gas flow with time betweenthe upper portion of the gas leg of the U-tube and the volume occupiedby the cement slurry being tested. The U-tube used had an internaldiameter of 1" and a length of 80" and was maintained at 72° F. Theexternally pressurized gas was nitrogen and was maintained at a pressurebetween 310 and 340 psig.

The curves A and B relate to the performance of two samples of aqueouspumpable slurries of Class H hydraulic cement. The slurries eachcontained 40% by weight of H₂ O, 2% by weight of calcium chloride(accelerator) and 1% by weight of HLX-C 249 (fluid loss control agentavailable from the Halliburton Company). The slurries tested differedonly in that the slurry represented by Curve A contained stoichiometricproportions of hydrazine and hydrogen peroxide sufficient for generating0.055 moles per liter of nitrogen gas. The initial similarity andsubsequent divergence of Curves A and B shows that the initial behaviorof both of the slurries was substantially identical throughout most ofthe first four hours. In both cases the externally pressurized gasflowed into the slurry at about the same rate.

Such an inflow of externally pressurized gas appears to be due to ashrinkage in the volume of the slurry. This may be due to portions ofthe mix water being utilized in hydrating the cement, and/or may be dueto some other type of conversion of slurry components to solids that areless voluminous.

It should be noted that initially, and throughout a significantproportion of this period, the cement slurry is enough like a liquid totransmit a hydrostatic pressure plus any externally applied pressure toany material contacted along the column of the slurry. Thus, forexample, in the annulus of a wellbore which encounters a subterraneangas sand, an entry or initial inflow of the externally pressurizedreservoir gas into the volume occupied by the cement slurry would be,ordinarily, prevented by the hydrostatic head existing within theadjacent portion of the column of cement slurry.

In the experiments shown in FIG. 2, the initial setting of each sampleof cement took place within about four hours. But, as indicated by theabrupt decrease (in Curve A) in the rate of the gas flow into the cementslurry containing the gas-generating reactants, the rate at which gaswas being internally generated was then accelerated to such an extentthat, for about the next four hours, little or no externally pressurizedgas was able to flow into the setting and set cememt. In fact, duringthat period there was some gas flow away from the cement (or cementslurry), and thus back into the body of externally pressurized gas(mainly in gas columns 12(a) and 12(b) of FIG. 1) due to gas beinggenerated internally at a rate slightly greater than needed to fill thevolume lost by the shrinking of the cement slurry. As shown by thelevelling off of the curve A after a curing time of about seven hoursthere was substantially no further gas flow into or out of the cementslurry that contained the gas-generating reactants. In contrast, asshown by curve B, in the conventional type of slurry there was a steadyinflow of gas into the slurry and that flow was continued (through avisible gap between the pipe wall and the solidified cement) after thesetting of the cement.

Shrinking Control at 72° F. and 340 psig

FIG. 3 shows a graph of the percent decrease with time of an aqueousslurry of hydraulic cement while the cement is setting. The cement usedand the test conditions are those described regarding FIG. 2.

Curve A shows the percentage of shrinking (which is, of course, directlyrelated to the amount of externally pressurized gas which flows into andout of the cement slurry) exhibited by such a slurry cntaining nogas-generating reactants. After about 20 hours, when that cement wasthoroughly set and hardened, oil was drained from the oil containing legof the U-tube apparatus of FIG. 1 so that the pressure on that leg ofthe column of cement was reduced to atmospheric. This subjected thecolumn of cement to a pressure gradient of about 340 psi and induced arelatively high rate of leaking amounting to 0.1 cc per minute per psiand caused a gap to be visible between the pipe wall and the cement.

Curve B shows the percentage shrinking of the same cement slurrycontaining hydrazine and hydrogen peroxide reactants for generating0.055 moles per liter of nitrogen gas. It shows that from about 4 to 8hours after the start of the test (when a flow of gas out of the cementslurry is shown on curve A of FIG. 2) the cement regained much of thevolume which was lost during the first 1 to 4 hours of the test. Thecement volume then became constant at a value such that there was novisible gap between the pipe wall and the cement. That body of cementpermitted no measurable leakage of gas (i.e., less than 1×10⁻⁶) inresponse to a pressure differential of 340 psi.

Gas Flow Control at 180° F. and 2000 psig

FIG. 4 shows a plot (similar to that of FIG. 2) of the flow of gas intoor out of the volumes occupied by columns of comparable cement slurrieswhile they are standing at 180° F. in contact with an externallypressurized gas at 2000 psi. Such tests were conducted as describedregarding FIG. 2.

The slurries used regarding FIG. 4 were comparable Class H cementslurries containing 40% by weight of water and conventional acceleratorsand fluid-loss retarders. From times starting substantially as soon asthe slurries were flowed into the U-tube 1 of the apparatus shown inFIG. 1, the slurries were each subjected to an externally pressurizedgas which was maintained at substantially 2000 psi (plus or minus about20 psi).

Curve A shows the performance of such a slurry containing 0.66 moles perliter of hydroxylamine hydrochloride plus 0.66 moles per liter of sodiumhydroxide (to neutralize the HCl). Curve B shows the performance ofsubstantially the same slurry without the gas-generating reactant.

Curve A shows that, during the first four hours, there was an initialgas flow into the volume occupied by each of the slurries. But, afterabout 4 hours, the rate at which gas was being generated within theslurry of Curve A became sufficiently rapid to prevent any furtherinflow of the externally pressurized gas into the volume initiallyoccupied by the slurry. Such an inflow was prevented throughout atesting period of about 40 hours.

On the other hand, curve B shows that the externally pressurized gasflowed continuously into the volume occupied by the slurry devoid of thegas-generating reactants. In addition, after allowing about 40 hours forthe setting and hardening of the cement, tests were made for the gasleakage through the U-tubes plugged by the slurries. A 100 psi (poundsper square inch) pressure differential was applied across the legs ofthe U-tubes. This caused a gas flow past the plug formed by the cementslurry of curve B at a rate of 0.22 cc per minute (with the volume ofthe gas being measured at 20° C. and 14.7 psi absolute). Under the sameconditions the cement slurry designated by curve A allowed no flow ofgas past the cement plug.

Gas Flow Control at 200° F. and 2000 psig

FIG. 5 shows a similar plot of similar flows of gas into and out ofcomparable slurries of Class H cement while the cement was being set at200° F. in contact with an externally pressurized gas being maintainedat substantially 2000 psig.

Curve A shows the performance of a cement slurry free of internalgas-generating reactants. After being allowed to set and cure for about65 hours, the cement plug formed by that slurry permitted the leakage ofgas across it at a rate of 0.015 cc per minute per pound per square inchin response to a 300 psi differential across the cement plug (with thevolume of the gas being measured at 2000 psig and 20° C.).

Curve B shows the performance of the same slurry modified by theaddition of an amount of hydrazine which is equivalent to 0.33 moles perliter of gaseous nitrogen, provided by the reaction, 3N₂ H₄ →N₂ +4NH₃.The resulting rate and amount of internal gas generation providedsufficient gas to reduce the leakage rate to 0.000014 cc (1.4×10⁻⁵ cc)per minute per psi of applied pressure differential.

Curve C shows the performance of the same slurry as that used in Curve Amodified by the inclusion of hydrazine, ferric hydroxide and a catalyticamount of copper sulfate in proportions sufficient to provide 0.33 to1.0 moles per liter hydrogen gas from a combination of the reactions:##STR1## The leakage prevention shown by curve C indicates that, with aninternal gas generating system which reacted slightly faster than thatof curve B, the externally pressurized gas flowed into the volumeoccupied by the slurry for about the first 4 hours and then, due to therate and volume of the internal gas generation, inflow of external gaswas first slowed and then stopped, within about 10 hours. For a periodof from about 10 to 70 hours after the start of the test, the rate ofgas generation was fast enough so that gas (probably inclusive of thatwhich had previously flowed into the slurry-occupied volume) wasgradually expelled back into the gas-occupied volume of the standpipe 6of the test apparatus. The cement plug formed by this slurry did notpermit any leakage of gas across it when a 300 psi differential wasapplied.

Shrinking Control at 250° F. and 5700 psig

FIG. 6 shows the graph of the percent decrease with time in the volumeof hydraulic cement slurries being set at 250° in contact with a gaspressurized at 5700 psig. The curve A cement was formed from an aqueousliquid slurry of class H hydraulic cement free of gas-generatingreactant. Within about 6 hours that slurry had shrunk by about 4% of itsvolume and, after about 20 hours, the leaking of the set and hardenedcement was found to be relatively high, about 10⁻⁴ cc per minute perpsig. The curve B cement had a composition which was the same except forthe inclusion of hydrazine, ferric oxide and cupric sulfate in a ratioof moles per liter of, 2 to 1 to 0.1, for producing 2 moles per liter ofnitrogen. The shrinkage of that slurry was only about 2% and its rate ofgas leakage was only 10⁻⁵ cc per minute per psi. The curve C cement hada composition which was the same as that of the curve B cement exceptfor a ratio of reactants of 3 to 1 to 0.1. The percent decrease in itsvolume was almost 0 and the leak rate was too small to be detected.

Controlling Gas Inflow at 140° to 180° F. and 2000 psig

FIG. 7 shows a graph of the volume change with time of a cement slurry(amounting to the gas flow with time into and out of the slurry) whilethe gas is pressurized at 2000 psig and the slurry is being set at thetemperature indicated. In each curve the slurry used was an aqueousslurry of Class H hydraulic cement having a pH of about 12 with andwithout an inclusion of 0.66 moles per liter of hydroxylamine (which isa nitrogen-generating reactant capable of providing 0.66 moles per literof nitrogen gas). As shown by the curves of volume change and the ratesof gas leakage after the cement had set and hardened at 140° F., interms of cc per minute per psi, the leak rate was about 0.002. At 160°F. it was about 40 times less. At 180° F., the leak rate wassubstantially zero with the gas-generating reactant--but, without thatreactant, the leak rate was 0.0055 cc per minute per psi.

These curves indicate the need for the gas-producing reactants to beboth (1) capable of providing enough gas to compensate for the shrinkageof the slurry and (2) capable of providing that gas at a rate which isproperly correlated with the temperature and pressure at which thecement is being cured (the magnitudes of which affect the rate of thehydrating and the like reactions which are involved in the setting ofthe cement).

Various Gas Leak Test Results

The enclosed Table lists the results of various tests conducted in themanner described above (in the apparatus of FIG. 1) at temperaturesranging from 75° to 291° F. and pressures of 340 to 4000 psig. In suchtests the cement slurries each contained Class H hydraulic cement anddiffered significantly only in the presence of the specified "reactantadditives" for accomplishing or assisting the generation of the nitrogengas. Each of the slurries contained from about 36 to 40% of water and 0or 2% calcium chloride accelerator and 0 or 1% of HLX-C 249 fluid losscontrol agent. But the latter variations in composition have been foundto cause little or no variation in the present type gas leak testresults.

                  TABLE 1                                                         ______________________________________                                                                             Gas Leak                                 Test Pressure Temp.   N.sub.2 H.sub.4                                                                     Reactant -                                                                             cc/Min/psi                               No.  (psig)   (°F.)                                                                          (M/L) Additives                                                                              0 = 1 × 10.sup.-6                  ______________________________________                                         1    340      75     0     0        1.7 × 10.sup.-2                     2    340      75     .075  .113 H.sub.2 O.sub.2                                                                   0                                         3    340     100     .075  .113 H.sub.2 O.sub.2                                                                   0                                         4    340     100     0     0        .sup. 9.7 × 10.sup.-12              5   1000      75     0     0        2.9 × 10.sup.-2                     6   1000      75     .225  .338 H.sub.2 O.sub.2                                                                   0                                         7   2000     120     0     0        1.0 × 10.sup.-2                     8   2000     120     .45   .68 H.sub.2 O.sub.2                                                                    0                                         9   2000     180     0     0        5.7 × 19.sup.-3                    10   2000     180     0     .66 NH.sub.2 OH                                                                        0                                        11   2000     200     0     .66 NH.sub.2 OH                                                                        0                                        12    340      75     .167  .22 K.sub.2 Cr.sub.2 O.sub.7                                                           0                                        13   1000     100     .167  .22 K.sub.2 Cr.sub. 2 O.sub.7                                                          0                                        14   2000     150     .167  .22 K.sub.2 Cr.sub.2 O.sub.7                                                           0                                        15   2000     200     .167  .22 K.sub.2 Cr.sub.2 O.sub.7                                                           0                                        16   2000     150     1.0   0          5 × 10.sup.-2                    17   2000     180     0     0        5.7 × 10.sup.-3                    18   2000     150     1.0   1.0 Fe.sub.2 O.sub.3 &                                                                 4.6 × 10.sup.-2                                                .05 CuSO.sub.4                                    19   2000     200     1.0   1.0 Fe.sub.2 O.sub.3 &                                                                 0                                                                    .05 CuSO.sub.4                                    20   2000     200     0     1.0 Fe.sub.2 O.sub.3 &                                                                 0                                                                    .05 CuSO.sub.4                                    21   2000     200     0     0 & .05 CuSO.sub.4                                                                     0                                        22   2000     200     0     0        1.3 × 10.sup.-2                                                0                                                 23   2000     200     1.0            1.7 × 10.sup.-5                    24   2000     200     1.0   1.0 Fe.sub.2 O.sub.3 &                                                                 0                                                                    .1 Cu.sub.2 O                                     25   2000     200     1.0   0        0                                                                    .1 Cu.sub.2 O                                     26   4000     200     0     0        6.7 × 10.sup.-6                                                0                                                 27   4000     200     1.0   1.0 Fe.sub.2 O.sub.3 &                                                                 0                                                                    .1 Cu.sub.2 O                                     28   4000     200     0     1.0 Fe.sub.2 O.sub.3                                                                   1.4 × 10.sup.-4                                                0                                                 29   4000     200     0     1.0 Fe.sub.2 O.sub.3 &                                                                 2.2 × 10.sup.-5                                                .1 Cu.sub.2 O                                     30   4000     200     1.0   1.0 Fe.sub.2 O.sub.3 &                                                                 1.5 × 10.sup.-5                                                .1 Cu.sub.2 O                                     31   4000     200     2.0   1.0 Fe.sub.2 O.sub.3 &                                                                 0                                                                    .1 Cu.sub.2 O                                     32   4000     266     2.0   1.0 Fe.sub.2 O.sub.3 &                                                                 0                                                                    .1 Cu.sub.2 O                                     33   4000     291     2.0   1.0 Fe.sub.2 O.sub.3 &                                                                 0                                                                    .1 Cu.sub.2 O                                     ______________________________________                                    

Note that tests of the type listed have involved anomalous resultsregarding the effect of copper. For example, in tests such as 18 to 22and 28 to 30 regarding the iron and copper catalyzed reaction ofhydrazine, it appears that when the hydrazine and the iron reactantswere omitted, the copper alone had a significant effect on the amount ofgas leakage which would occur. Such tests indicate that significantleakage prevention can be obtained by copper sulfate or cuprous oxide inconcentrations of from about 0.5 to 0.7 moles per liter.

In general, the cement slurry-shrinkage and cement gas-leakage tests inaccordance with this invention indicate the following regarding thesuitability of a given cement slurry composition for use in forming aset cement at a given pressure and temperature. As temperature increaseswithin a relatively low temperature range, so does the tendency for thecement to leak gas. Temperature usually has little or no effect aboveabout 2000 psi. At temperatures above 120° F., increases in pressuretend to cause decreases in leak-rate, up to a significantly highpressure (e.g., probably between about 3000 and 4000 psi), and furtherincreases in pressure have little or no effect. In the present type ofgas-stabilization of cement slurries, where the gas-generating reactantsare solutes within the aqueous liquid phase of the slurry, the presenttests indicate that the sealing is better when the generation of asignificant amount of gas is continued from about 10 to 20 hours, inorder to prevent most of the shrinking of the slurry.

The approximate amount of gas which can be or need be included within agas-stabilized cement which is formed at a given pressure andtemperature can be readily calculated. For example, where hydrazine andhydrogen peroxide are the reactants, one mole of gaseous nitrogen isyielded by each mole of hydrazine. So, for example, assume that 40 gramsof water (containing 0.1 m/l N₂ H₄ and 0.2 m/l H₂ O₂) will be mixed with60 grams of cement (volume of cement=16 cc), when the reaction in thewater is complete, 0.1 m/l of N₂ will form. For the total volume ofcement slurry (40 cc H₂ O+16 cc cement=56 cc), we have ##EQU1## Since 1mol. of N₂ gas at 20° C., and 14.7 psi=24 liters gas, we calculate thefollowing table:

    ______________________________________                                        Pressure                                                                      on Gas Volume of Gas Total Volume   % Gas                                     psi    at 29° C., L                                                                         of slurry      in Slurry                                 ______________________________________                                        14.7   .004 × 24 = .094                                                                      .094 + .056 = .150                                                                           63                                        (1 bar)                                                                       147    .004 × 2.4 = .0094                                                                    .0094 + .056 = .0654                                                                         14                                        (10 bar)                                                                      1470   .004 × .24 = .00094                                                                   .00094 + .056 = .0569                                                                        1.7                                       (100 bar)                                                                     ______________________________________                                    

Thus, at a subterranean depth of about 3000 feet (at a pressure of 200bar) the percentage of gas introduced into the cement would be less thanabout 1% of its volume. The suitability of such a cement slurry for usein a particular cement region at a particular pressure and temperaturecan be readily confirmed by the presently described cement slurryshrinking and leaking tests.

Suitable Compositions and Procedures

Suitable cements include substantially any cementous materials used inthe form of an aqueous slurry containing suspended inorganic materialwhich sets or cures to form an integral solid and relatively impermeablemass in a manner such that a significant proportion of the aqueousliquid is consumed by a conversion such as hydration reaction orsorption on solid materials during the formation of that solidifiedmass. The present invention is particularly useful in forming solidifiedhydraulic cements, particularly where the cement slurry containssubstantially all, but not much more than, the amount of liquid that isso-consumed during the setting of the cement. Where the slurry is to bepositioned in contact with porous materials such as subterranean earthformations, such slurries can advantageously contain fluid-lossmaterials such as those conventionally used in oil well cements.Similarly, such cement slurries can contain accelerators and/orretarders such as those conventionally used in oil well cements. Thenitrogen gas-forming reactants should of course be selected so as to becompatible with all of the cement-forming compounds and additives whichare to be included in the cement slurry.

Reactants suitable for generating the nitrogen gas in accordance withthe present process can be substantially any which are water-soluble andcapable of reacting while they are dissolved in an aqueous solution toproduce nitrogen gas and by-products which are substantially inert tothe components of an aqueous slurry of cement and the setting of thecement. Particularly suitable reactants are members of the groupconsisting of a mixture of hydrazine and hydrogen peroxide,hydroxylamine, a mixture of hydroxylamine and a water soluble chromate,a mixture of hydroxylamine and hydrogen peroxide, and a mixture ofhydrazine and ferric hydroxide with a catalyst of chromium or coppersalts.

With respect to the reactants, two types of concentrations areimportant. The concentration of the reactants within the aqueous liquidphase of the cement slurry affects the rate at which the reactantsgenerate the gas at a given temperature. In general, the reaction rateincreases with increases in concentration. The concentration of thereactants within the slurry as a whole affects the amount and thus thevolume of the gas that is generated. The concentration within the slurryas a whole is affected by both the proportion of the reactants in theaqueous liquid and the proportion of the aqueous liquid in the slurry.For example, if the slurry contains only about 36% by weight of water, agreater concentration of reactants in the aqueous liquid is needed inorder to generate a given amount of gas within the cement slurry thanwould be needed if the water content of the slurry were 40%. If theconcentration of the reactants in the aqueous liquid is relatively high(e.g., in order to provide an adequate amount of reactant within theslurry as a whole), the kind of reactants and/or solution pH orcatalyst, or the like, should be adjusted to compensate for therelatively rapid rate of reaction provided by the relatively highconcentration.

What is claimed is:
 1. A cement slurry testing apparatus comprising:atest chamber having a substantially rigid-walled generally tubularportion; means for flowing a sample of cement slurry into the chamber sothat it fills at least a transverse section of said tubular portion;means for inflowing and pressurizing a gas within the chamber so that abody of gas contacts and presses against said inflowed cement slurry;means including a pump for maintaining the so-positioned gas and cementslurry within the chamber at a selected pressure and temperature; and,means for measuring the amount of said pressurized gas which flows intoor away from the transverse section of said chamber that was initiallyoccupied by the cement slurry.
 2. The apparatus of claim 1 in which saidtransverse section of the test chamber is located in a bottom portion ofa generally U-shaped tube with upwardly extending legs.